Cesium and lithium-containing quaternary compound scintillators

ABSTRACT

The present invention relates to quaternary compound scintillators and related devices and methods. The scintillators may include, for example, a mixed halide scintillator composition including at least two different CsLiLa halide compounds and a dopant. Related detection devices and methods are further included.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.13/089,212, entitled “Cesium and Lithium-Containing Quaternary CompoundScintillators,” filed Apr. 18, 2011, which is a continuation of U.S.application Ser. No. 12/405,137, entitled, “Cesium andLithium-Containing Quaternary Compound Scintillators,” filed on Mar. 16,2009, which is a continuation-in-part of U.S. application Ser. No.11/938,176, entitled “Cesium and Lithium-Containing Quaternary CompoundScintillators,” filed on Nov. 9, 2007 and claims the benefit of priorityto U.S. Application No. 61/045,849, filed Apr. 17, 2008, the entirecontent of each of which are incorporated herein by reference.

The present application is related to U.S. application Ser. No.12/405,155, entitled “Cesium and Sodium-Containing ScintillatorCompositions,” filed on Mar. 16, 2009 and U.S. application Ser. No.12/405,168, entitled “Mixed Cesium Sodium and Lithium HalideScintillator Compositions,” filed on Mar. 16, 2009, the full disclosuresof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a scintillator and related devices andmethods. More specifically, the present invention relates toscintillators including a cesium and lithium-containing quaternarycompound and a dopant for use, for example, in radiation detection,including gamma-ray spectroscopy, and X-ray and neutron detection,and/or imaging applications.

Scintillation spectrometers are widely used in detection andspectroscopy of energetic photons (e.g., X-rays, gamma-rays, etc.). Suchdetectors are commonly used, for example, in nuclear and particlephysics research, medical imaging, diffraction, non destructive testing,nuclear treaty verification and safeguards, nuclear non-proliferationmonitoring, and geological exploration.

Important requirements for the scintillation crystals used in theseapplications include high light output, transparency to the light itproduces, high stopping efficiency, fast response, good proportionality,low cost and availability in large volume. These requirements on thewhole cannot be met by many of the commercially available scintillators.While general classes of chemical compositions may be identified aspotentially having some attractive scintillation characteristic(s),specific compositions/formulations having both scintillationcharacteristics and physical properties necessary for actual use inscintillation spectrometers and various practical applications haveproven difficult to predict. Specific scintillation properties are notnecessarily predictable from chemical composition alone, and preparingeffective scintillators from even candidate materials often provesdifficult. For example, while the composition of sodium chloride hadbeen known for many years, the invention by Hofstadter of a highlight-yield and conversion efficiency scintillator from sodium iodidedoped with thallium launched the era of modern radiation spectrometry.More than half a century later, thallium doped sodium iodide, in fact,still remains one of the most widely used scintillator materials. Sincethe invention of NaI(Tl) scintillators in the 1940's, for half a centuryradiation detection applications have depended to a significant extenton this material. The fields of nuclear medicine, radiation monitoring,and spectroscopy have grown up supported by NaI(Tl). Although far fromideal, NaI(Tl) was relatively easy to produce for a reasonable cost andin large volume. With the advent of X-ray CT in the 1970's, a majorcommercial field emerged as did a need for different scintillators, asNaI(Tl) was not able to meet the requirements of CT imaging. Later, thecommercialization of positron emission tomography (PET) imaging providedthe impetus for the development of yet another class of detectormaterials with properties suitable for PET. As the methodology ofscintillator development evolved, new materials have been added, andyet, specific applications are still hampered by the lack ofscintillators suitable for particular applications.

As a result, there is continued interest in the search for newscintillators and formulations with both the enhanced performance andthe physical characteristics needed for use in various applications.Today, the development of new scintillators continues to be as much anart as a science, since the composition of a given material does notnecessarily determine its properties as a scintillator, which arestrongly influenced by the history (e.g., fabrication process) of thematerial as it is formed. While it may be possible to reject a potentialscintillator for a specific application based solely on composition, itis typically difficult to predict whether even a material with apromising composition can be used to produce a useful scintillator withthe desired properties.

One of the uses of radiation monitoring devices is preventing the spreadof weapons such as nuclear weapons. One way to passively determine thepresence of nuclear weapons is to detect and identify the characteristicsignatures of special nuclear materials (SNMs) such as highly enricheduranium and weapons grade plutonium. Characteristic X-rays andgamma-rays are signatures of these materials. The general approach to apassive gamma-ray assay is to acquire raw spectra, correct the spectrafor rate-related electronic losses and source attenuation, and computethe total corrected count rate, which is proportional to the mass of theisotope being assayed. The proportionality constant includes the effectsof gamma-ray emission rate, solid angle, and detector efficiency.

Monitoring for both highly enriched uranium and weapons grade plutoniuminvolves analysis of X-ray and gamma-ray spectra with multiple energiesof interest. One important consideration in SNM monitoring is thedetermination of uranium enrichment since highly enriched uranium can beused for development of nuclear weapons. The naturally occurringisotopic abundance of uranium is: ²³⁸U (99.27%), ²³⁵U (0.720%) and ²³⁴U(0.006%). When the fraction of the fissile ²³⁵U is higher than that innaturally occurring uranium, the uranium is said to be enriched. Therelative intensity of 185.7 keV gamma-rays (from ²³⁵U) compared to the94-98 keV X-rays for uranium in an unattenuated spectrum can be used todetermine uranium enrichment (Passive Nondestructive Assay of NuclearMaterials, eds. Reilly et al., U.S. Nuclear Regulatory Commission,Washington D.C., pp. 11-18, (1991)). As the uranium enrichment levelincreases, the relative intensity of the 185.7 keV gamma-ray peakincreases in comparison to the 94-98 keV X-ray peak. Anotherconsideration in SNM monitoring is to distinguish “weapons-grade”plutonium (with ≧93% ²³⁹Pu) from “reactor-grade” plutonium <60% ²³⁹Pu).The “reactor-grade” plutonium includes other isotopes such as ²⁴⁰Pu,²⁴¹Pu, ²⁴²Pu, and ²³⁸Pu. Comparison of gamma-ray signatures of ²³⁹Pu(such as 129.3 keV and 413.7 keV photons) with those for other plutoniumisotopes allows determination of the grade of plutonium. In addition tothe characteristic X- and gamma-rays of highly enriched uranium andweapons grade plutonium, there is considerable interest in themeasurement of irradiated fuel from nuclear reactors because of theplutonium produced during reactor operation. Due to very intensegamma-rays emitted by fission products of the irradiated fuel,gamma-rays from spontaneous decay of plutonium and uranium (²³⁵U, ²³⁹Puand ²⁴¹Pu) are generally not used for measurement of irradiated fuel.The most commonly measured fission product gamma-ray is the 662 keV linefrom ¹³⁷Cs. Recent threat of “dirty bombs” (devices which spreadradioactive material using conventional, non-nuclear explosives) hasalso created an interest in monitoring of radioactive materials such as¹³⁷Cs, ⁶⁰Co, ²⁴¹Am, radioactive medical waste, and irradiated fuel fromnuclear reactors that emit high energy gamma-rays.

Thus, gamma-ray spectrometers and radiation detectors are importanttools in monitoring of special nuclear materials. A number of securitysystems such as hand-held radioisotope identifiers, vehicle portals forradiation detection, and personal radiation detection devices rely onavailability of high performance gamma-ray spectrometers. Similarsystems are also required for nuclear non-proliferation monitoring. Animportant challenge in security monitoring is not only to detect hiddenradioactive materials but also to distinguish them from routinely usedradiopharmaceuticals as well as from naturally occurring benignradioactive materials.

Existing scintillator materials and commercial radiation detectors donot meet the current needs for radiation monitoring and weaponsdetection. For example, existing scintillators and detectors typicallylack the one or more important scintillation properties (e.g., highenergy resolution, light output, stopping power, fast response, and thelike) that are desired and/or are not useful in detecting both energeticphotons (e.g., gamma-rays and X-rays) as well as neutron emission. Thus,a need exists for improved scintillator compositions suitable for use invarious radiation detection applications, including, for example,radiation and nuclear weapons monitoring.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a scintillator composition including acesium and lithium-containing quaternary compound and a dopant, andrelated devices and methods. Exemplary scintillator compositions of theinvention can include Cs₂LiLaBr₆, Cs₂LiLaCl₆, Cs₂LiLaI₆, and Cs₂LiLuI₆.Scintillator compositions can include one or more of the cesium andlithium-containing quaternary compounds disclosed herein, and/or mayinclude a mixture of any two or more of the quaternary compoundsprovided. For example, a composition can include a mixed halidecomposition that includes at least two different CsLiLa halidecompounds. In one embodiment, halides can be selected from, for example,F, Cl, Br, or I.

Excellent scintillation properties, including high light output, goodproportionality, fast response, and good energy resolution have beenmeasured for compositions of the present invention, which weredemonstrated to be suitable for gamma-ray spectroscopy, as well as X-rayand neutron emission detection. Scintillator compositions describedherein are easy to grow have high light output and fast response. Forgamma rays, the non-proportionality is particularly outstanding and cantranslate into good energy resolution. Scintillator compositions of thepresent invention demonstrated very good non-proportionality (e.g.,about 4% non-proportionality over 60 to 1000 keV energy range), as wellas light output for neutrons that is very high compared to previouslyexisting neutron scintillators, and pulse shape discrimination betweenneutrons and gamma rays. Thus, compositions described hereinadditionally advantageously combine excellent gamma-ray detectionproperties along with neutron detection.

Scintillation properties of the certain crystals/compositions of thepresent invention and described herein include peak emission wavelengthsfrom 375 nm to 475 nm, which is well matched to PMTs as well as silicondiodes used in nuclear instrumentation and a peak wavelength forgamma-ray spectroscopy. The principal decay-time constant in oneinstance was measured at approximately 90 ns, which is faster than thedecay-time constant of commercial scintillators such as BGO (see, e.g.,Table 1 infra). Thus, in one embodiment, the present invention includesscintillator compositions having decay time constants better than, orhaving a value less than, 150 ns. In some embodiments, the decay timeconstant can be better than 100 ns, or even better than 50 ns. Undergamma ray excitation, the light output can be greater than 10,000photons/MeV and, in some embodiments, greater than 40,000 photons/MeV,which is greater than that of many commercial scintillators.

One aspect of the present invention includes a scintillator including aquaternary compound. Scintillators can include a quaternary compoundhaving Cs in the first position, Li in the second position, La or Lu inthe third position, and Cl, Br, or I in the fourth position. In certainembodiments, the scintillator can include a single dopant or a mixtureof dopants. Scintillator compositions of the present invention caninclude a mixture of different compounds (e.g., quaternary compounds)disclosed herein, such as mixed halide compositions including two ormore different CsLiLa halide compounds.

In another aspect, the invention further includes devices, such as aradiation detection device having a scintillator including a quaternarycompound and a dopant, and a photodetector assembly optically coupled tothe scintillator. The photodetector assembly can include, for example, aphotomultiplier tube, a photo diode, or a PIN detector. The device mayfurther include a data analysis, or computer, system for processing andanalyzing detected signals.

In yet another aspect, the invention includes an X-ray and neutrondetector assembly, including a scintillator including a quaternarycompound and a dopant, a photodetector assembly, and electronics or asystem for data processing/analysis. For example, the device can includeelectronics configured for performing pulse-shape analysis todifferentiate gamma ray from neutron emissions.

In yet another aspect, the invention includes a method of performingradiation detection. Such a method can include, for example, providing adetection device having a detector assembly including a scintillatorincluding a quaternary compound and a dopant; a photodetector assembly;and positioning a target within a field of view of the scintillator asto detect emissions from the target. Emissions can include, for example,gamma-ray, X-ray, or neutron emissions. A target can include variouspotential sources of detectable emissions including neutron emitters(e.g., plutonium and the like), gamma-ray sources (e.g., uranium and thelike), X-ray sources, etc. In one embodiment, for example, thescintillator compositions can be used for imaging applications includingmedical imaging such as in a method of performing PET (e.g.,time-of-flight PET) or SPECT. In such an embodiment, the imaging methodcan comprise injecting or otherwise administering a patient with adetectable label, and, after a sufficient period of time to allowlocalization or distribution of the label, placing the patient withinthe field of view of the detection device. Thus, in some embodiments thetarget includes a patient or a portion of a patient's body.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be had to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings. The drawingsrepresent embodiments of the present invention by way of illustration.The invention is capable of modification in various respects withoutdeparting from the invention. Accordingly, the drawings/figures anddescription of these embodiments are illustrative in nature, and notrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1D depict ¹³⁷Cs spectra collected with a Cs₂LiLaBr₆:Ce(FIG. 1A), Cs₂LiLaCl₆:Ce (FIG. 1B), Cs₂LiLaI₆:0.5% Ce (FIG. 1C), andCs₂LiLuI₆:2% Ce (FIG. 1D) scintillators, each coupled to a PMT. Theenergy resolution of a 662 keV peak for Cs₂LiLaBr₆:Ce, Cs₂LiLaCl₆:Ce,Cs₂LiLaI₆:0.5% Ce, Cs₂LiLuI₆:2% Ce are about 5.9%, 6.1%, 6.2%, and 19%(FWHM), respectively. FIGS. 1A through 1C further depict ¹³⁷Cs spectrafor BGO coupled to a PMT.

FIGS. 2A and 2B depict ²⁵²Cf and ¹³⁷Cs spectra for Cs₂LiLaBr₆:Ce (FIG.2A) and Cs₂LiLaCl₆:Ce (FIG. 2B).

FIGS. 3A through 3D depict optical emission spectra for Cs₂LiLaBr₆:Ce(FIG. 3A), Cs₂LiLaCl₆:Ce (FIG. 3B), Cs₂LiLaI₆:0.5% Ce (FIG. 3C), andCs₂LiLuI₆:Ce (FIG. 3D) scintillators upon exposure to X-rays.

FIGS. 4A through 4D depict time profiles for Cs₂LiLaBr₆:Ce (FIG. 4A),Cs₂LiLaCl₆:Ce (FIG. 4B), Cs₂LiLaI₆:Ce (FIG. 4C), and Cs₂LiLuI₆:2% Ce(FIG. 4D) scintillators exposed to gamma rays. Rise and decay times aretabulated in Table 1.

FIGS. 5A and 5B show time profiles for Cs₂LiLaBr₆:Ce and Cs₂LiLaCl₆:Cescintillators, respectively, where the scintillators exhibit differentdecay times for gamma rays and neutrons. The sharp spike superimposed onthe rising edge may be an electronic artifact.

FIGS. 6A through 6C illustrate proportionality of response forCs₂LiLaBr₆:1% Ce (FIG. 5A), Cs₂LiLaCl₆:Ce (FIG. 5B), and Cs₂LiLaI₆:0.2%Ce (FIG. 5C) scintillator compositions. The figures show light output ofthe quaternary compound scintillators measured under excitation fromisotopes such as ²⁴¹Am (60 keV γ-rays), ⁵⁷Co (14.4, 122, and 136 keVγ-rays), ²²Na (511 keV and 1275 keV γ-rays) and ¹³⁷Cs (662 keV γ-rays).

FIG. 7 is a conceptual diagram of a detector assembly of the presentinvention.

FIG. 8 depicts an emission spectrum for a mixed halide composition,Cs₂LiLaCl_(4.8)Br_(1.2):Ce, according to one exemplary embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention will be better understood with resort to the followingdefinitions:

A. Rise time, in reference to a scintillation crystal material, shallmean the speed with which its light output grows once a gamma-ray hasbeen stopped in the crystal. The contribution of this characteristic ofa scintillator combined with the decay time contribute to a timingresolution.

B. A Fast timing scintillator (or fast scintillator) typically includesa timing resolution of about 500 ps or less. For certain PETapplications (e.g., time-of-flight (TOF)), the fast scintillator shouldbe capable of localizing an annihilation event as originating fromwithin about a 30 cm distance, i.e., from within a human being scanned.

C. Timing accuracy or resolution, usually defined by the full width halfmaximum (FWHM) of the time of arrival differences from a point source ofannihilation gamma-rays. Because of a number of factors, there is aspread of measured values of times of arrival, even when they are allequal. Usually they distribute along a bell-shaped or Gaussian curve.The FWHM is the width of the curve at a height that is half of the valueof the curve at its peak.

D. Light output shall mean the number of light photons produced per unitenergy deposited by the detected gamma-ray, typically the number oflight photons/MeV.

E. Stopping power or attenuation shall mean the range of the incomingX-ray or gamma-ray in the scintillation crystal material. Theattenuation length, in this case, is the length of crystal materialneeded to reduce the incoming beam flux to 1/e⁻.

F. Proportionality of response (or linearity). For some applications(such as CT scanning) it is desirable that the light output besubstantially proportional to the deposited energy. For single photonspectroscopy, non-proportionality of response is an important parameter.In a typical scintillator, the number of light photons produced per MeVof incoming gamma-ray energy is not constant. Rather, it varies with theenergy of the stopped gamma-ray. This has two deleterious effects. Thefirst is that the energy scale is not linear, but it is possible tocalibrate for the effect. The second is that it degrades energyresolution. To see how this occurs, consider a scintillator thatproduces 300 photons at 150 keV, 160 photons at 100 keV and 60 photonsat 50 keV. From statistics alone, the energy resolution at 150 keVshould be the variability in 300 photons, which is 5.8%, or 8.7 keV. Ifevery detected event deposited 150 keV in one step this would be thecase. On the other hand, if, as it occurs, an event deposited 100 keV ina first interaction and then another 50 keV in a second interaction, thenumber of photons produced would not be 300 on the average, but160+60=220 photons, for a difference of 80 photons or 27%. In multipledetections, the peak would broaden well beyond the theoretical 8.7 keV.The smaller the non-proportionality the smaller this broadening and thecloser the actual energy resolution approaches the theoretical limit.

The quaternary compound scintillator material of the present inventionwill respond by emitting light after detecting charged particles, highenergy photons, and neutrons, thereby providing useful scintillationproperties. The quaternary compound scintillator has a compositionincluding Cs in the first position, Li in the second position, La or Luin the third position, and Cl, Br, or I in the fourth position.Scintillator compositions can further include a dopant. In certainembodiments, the elements exist in atomic ratios of 2:1:1:6, such asCs₂LiLaBr₆.

Scintillator compositions can include a single quaternary compoundselected from those described herein or a mixture thereof. Compositionscan, e.g., include mixed halide compositions with at least two differentCsLiLa halide compounds. Thus, one advantage of the present inventionincludes the ability to select or customize a particular mixedcomposition ratio or formulation that may be selected at least partiallybased on intended use of the scintillator composition and/or desiredproperties or scintillation/performance characteristics. For example, aparticular mixed quaternary compound composition (e.g., mixed CsLiLahalide composition) formulation may be selected based on one or more ofa variety of factors, such as desired stopping power (e.g., at leastpartially as a function of atomic number), neutron response, bandgaplevels tuning/optimization for Ce emission and light output, pulse shapediscrimination, gamma-ray sensitivity, and the like.

In one embodiment, a mixed halide scintillator composition can include,for example, a mixture of two or more of Cs₂LiLaF₆, Cs₂LiLaCl₆,Cs₂LiLaBr₆, and Cs₂LiLaI₆. As noted above, combinations are not limitedto any specific halide ratio/formulation for mixed halide scintillatorcompositions. FIG. 8 shows an emission spectrum of an exemplary mixedhalide composition in which Cl and Br are present at a ratio of 4.8:1.2.

The scintillators of the invention are particularly useful, for example,for spectroscopy detection of energetic photons (e.g., X-rays,gamma-rays), as well as for neutron emission detection. Notablecharacteristics for the scintillation compositions of the inventioninclude surprisingly robust light output, high gamma-ray and neutronstopping efficiency (attenuation), fast response, and goodnon-proportionality. Furthermore, the scintillator compositions can beefficiently and economically produced. Thus, detectors having aquaternary compound scintillator that is described in the presentinvention are useful in a wide variety of applications, includingwithout limitation nuclear and high energy physics research, medicalimaging, diffraction, non-destructive testing, nuclear treatyverification and safeguards, and geological exploration.

The scintillator composition of the present invention can optionallyinclude a “dopant”. Dopants can affect certain properties, such asphysical properties (e.g., brittleness, etc.) as well as scintillationproperties (e.g., luminescence, etc.) of the scintillator composition.The dopant can include, for example, cerium (Ce), praseodymium (Pr),lutetium (Lu), lanthanum (La), europium (Eu), samarium (Sm), strontium(Sr), thallium (Tl), chlorine (Cl), fluorine (F), iodine (I), andmixtures of any of the dopants. The amount of dopant present will dependon various factors, such as the application for which the scintillatorcomposition is being used; the desired scintillation properties (e.g.,emission properties, timing resolution, etc.); and the type of detectiondevice into which the scintillator is being incorporated. For example,the dopant is typically employed at a level in the range of about 0.1%to about 20%, by molar weight. In certain embodiments, the amount ofdopant is in the range of about 0.1% to about 100%, or about 0.1% toabout 5.0%, or about 5.0% to about 20%, by molar weight.

The scintillator composition of the invention may be prepared in severaldifferent forms. In some embodiments, the composition is in acrystalline form (e.g., monocrystalline). Scintillation crystals, suchas monocrystalline scintillators, have a greater tendency fortransparency than other forms. Scintillators in crystalline form (e.g.,scintillation crystals) are often useful for high-energy radiationdetectors, e.g., those used for gamma-ray or X-ray detection. However,the composition can include other forms as well, and the selected formmay depend, in part, on the intended end use of the scintillator. Forexample, a scintillator can be in a powder form. It can also be preparedin the form of a ceramic or polycrystalline ceramic. Other forms ofscintillation compositions will be recognized and can include, forexample, glasses, deposits, vapor deposited films, microcolumnar, orother forms suitable for radiation detection as described herein. Itshould also be understood that a scintillator composition might containsmall amounts of impurities. Also, minor amounts of other materials maybe purposefully included in the scintillator compositions to affect theproperties of the scintillator compositions.

Methods for making crystal materials can include those methods describedherein and may further include other techniques. Typically, theappropriate reactants are melted at a temperature sufficient to form acongruent, molten composition. The melting temperature will depend onthe identity of the reactants themselves (see, e.g., melting points ofreactants), but is usually in the range of about 300° C. to about 1350°C. Non-limiting examples of the crystal-growing methods can includecertain techniques of the Bridgman-Stockbarger methods; the Czochralskimethods, the zone-melting methods (or “floating zone” method), thevertical gradient freeze (VGF) methods, and the temperature gradientmethods. See, e.g., Example 1 infra. (see also, e.g., “LuminescentMaterials”, by G. Blasse et al, Springer-Verlag (1994) and “CrystalGrowth Processes”, by J. C. Brice, Blackie & Son Ltd (1986)).

In the practice of the present invention, attention is paid to thephysical properties of the scintillator material. In particularapplications, properties such as hygroscopy (tendency to absorb water),brittleness (tendency to crack), and crumbliness should be minimal.

TABLE I Properties of Scintillators Light Principal Output Peak Rise-Decay (Photons/ Density Emission time Time Material MeV) (g/cm³) (nm)(ns) (ns) NaI(Tl) 38,000 3.67 415 >10 230 CsI(Tl) 52,000 4.51 540 >101000  LSO 24,000 7.4 420 <1  40 BGO  8,200 7.13 505 >1 300 BaF₂ 10,000~4.88 310, slow <0.1  620, 2,000 fast 220, fast slow 0.6, fast GSO  7,6006.7 430 ~8  60 CdWO₄ 15,000 8.0 480 5000  YAP 20,000 5.55 370 <1  26Cs₂LiLaBr₆:Ce 46,000 ~4.1 393 0.5, 12 1, 65 Cs₂LiLaCl₆:Ce 34,500 ~3.4377 0.5, 16 1.5, 90   Cs₂LiLaI₆:Ce 47,500 ~4.3 437 2  70 Cs₂LiLuI₆:Ce11,000 ~4.8 470 2 5, 45

Table I provides a listing of certain properties of a number ofscintillators. Compared to other commercially available scintillators,including CsI, which is among the scintillation materials with thehighest known light output, the compositions of these inventions producecomparable light output. In addition, they have a fast principal decayconstant.

As set forth above, scintillator compositions of the present inventionmay find use in a wide variety of applications. In one embodiment, forexample, the invention is directed to a method for detecting energyradiation (e.g., gamma-rays, X-rays, neutron emissions, and the like)with a scintillation detector including the scintillation composition ofthe invention.

FIG. 6 is a schematic diagram of a detector assembly of the presentinvention. The detector 10 includes a scintillator 12 optically coupledto a light photodetector 14 or imaging device. The detector assembly 10can include a data analysis, or computer, system 16 to processinformation from the scintillator 12 and light photodetector 14. In use,the detector 10 detects energetic radiation emitted form a source 18.

A data analysis, or computer, system thereof can include, for example, amodule or system to process information (e.g., radiation detectioninformation) from the detector/photodetectors can also be included in aninvention assembly and can include, for example, a wide variety ofproprietary or commercially available computers, electronics, or systemshaving one or more processing structures, a personal computer,mainframe, or the like, with such systems often comprising dataprocessing hardware and/or software configured to implement any one (orcombination of) the method steps described herein. Any software willtypically comprise machine readable code of programming instructionsembodied in a tangible media such as a memory, a digital or opticalrecording media, optical, electrical, or wireless telemetry signals, orthe like, and one or more of these structures may also be used totransmit data and information between components of the system in any ofa wide variety of distributed or centralized signal processingarchitectures.

The detector assembly typically includes material formed from thescintillator composition described herein (e.g., one or morescintillator crystals). The detector further can include, for example, alight detection assembly including one or more photodetectors.Non-limiting examples of photodetectors include photomultiplier tubes(PMT), photodiodes, CCD sensors, image intensifiers, and the like.Choice of a particular photodetector will depend in part on the type ofradiation detector being fabricated and on its intended use of thedevice. In certain embodiments, the photodetector may beposition-sensitive.

The detector assemblies themselves, which can include the scintillatorand the photodetector assembly, can be connected to a variety of toolsand devices, as mentioned previously. Non-limiting examples includenuclear weapons monitoring and detection devices, well-logging tools,and imaging devices, such as nuclear medicine devices (e.g., PET).Various technologies for operably coupling or integrating a radiationdetector assembly containing a scintillator to a detection device can beutilized in the present invention, including various known techniques.

The detectors may also be connected to a visualization interface,imaging equipment, or digital imaging equipment (e.g., pixilated flatpanel devices). In some embodiments, the scintillator may serve as acomponent of a screen scintillator. For example, powdered scintillatormaterial could be formed into a relatively flat plate, which is attachedto a film, such as photographic film. Energetic radiation, e.g., X-rays,gamma-rays, neutron, originating from a source, would interact with thescintillator and be converted into light photons, which are visualizedin the developed film. The film can be replaced by amorphous siliconposition-sensitive photodetectors or other position-sensitive detectors,such as avalanche diodes and the like.

Imaging devices, including medical imaging equipment, such as the PETand SPECT devices, and the like, represent another important applicationfor invention scintillator compositions and radiation detectors.Furthermore, geological exploration devices, such as well-loggingdevices, were mentioned previously and represent an importantapplication for these radiation detectors. The assembly containing thescintillator usually includes, for example, an optical window at one endof the enclosure-casing. The window permits radiation-inducedscintillation light to pass out of the scintillator assembly formeasurement by the photon detection assembly or light-sensing device(e.g., photomultiplier tube, etc.), which is coupled to the scintillatorassembly. The light-sensing device converts the light photons emittedfrom the scintillator into electrical pulses that may be shaped anddigitized, for example, by the associated electronics. By this generalprocess, gamma-rays can be detected, which in turn provides an analysisof geological formations, such as rock strata surrounding the drillingbore holes.

In many of the applications of a scintillator composition as set forthabove (e.g., nuclear weapons monitoring and detection, imaging, andwell-logging and PET technologies), certain characteristics of thescintillator are desirable, including high light output, fast rise timeand short decay time, good timing resolution, and suitable physicalproperties. The present invention is expected to provide scintillatormaterials which can provide the desired high light output and initialphoton intensity characteristics for demanding applications of thetechnologies. Moreover, the invention scintillator compositions are alsoexpected to simultaneously exhibit the other important properties notedabove, e.g., fast rise time, short decay time, good stopping power, andtiming resolution. Furthermore, the scintillator materials are alsoexpected to be produced efficiently and economically, and also expectedto be employed in a variety of other devices which requireradiation/signal detection (e.g., gamma-ray, X-ray, neutron emissions,and the like).

The following examples are intended to illustrate but not limit theinvention.

EXAMPLES Example 1

The present example provides a method for growing and providescharacterization for quaternary compound scintillator crystals. Thefollowing examples are offered by way of illustration, not by way oflimitation.

Crystal Growth of Cs₂LiLaBr₆:Ce, Cs LiLaCl₆:Ce Cs LiLaI₆:Ce andCs₂LiLuI₆: Ce

In one example, a one zone Bridgman furnace was used for crystal growth.Typical growth rates for the Bridgman process are about 10 mm/hour.Growth rates ranging from about 1 mm/day to about 1 cm/hour may beutilized. Rates of about 5 to about 30 mm/day can typically be used. Therange of rates may be extended to improve material quality.

Cs₂LiLaBr₆, Cs₂LiLaCl₆, and Cs₂LiLaI₆ have a cubic crystal structure.Cs₂LiLuI₆ has a trigonal crystal structure. The density of Cs₂LiLaBr₆,Cs₂LiLaCl₆, Cs₂LiLaI₆, and Cs₂LiLuI₆ is 4.12, 3.36, between 4.5 and 5,and 4.77 g/cm³, respectively. Cs₂LiLaBr₆, Cs₂LiLaCl₆, Cs₂LiLaI₆, andCs₂LiLuI₆ melt congruently at approximately 788, 859, 778 and 1050° C.,respectively, and therefore their crystals can be grown using melt basedmethods such as those described by Bridgman and Czochralski. Thesemelt-based processes are well suited for growth of large volume crystals(Brice, Crystal Growth Processes, Blackie Halsted Press (1986)). TheBridgman method has been used for growing Cs₂LiLaBr₆, Cs₂LiLaCl₆,Cs₂LiLaI₆, and Cs₂LiLuI₆. Both the vertical and horizontal orientationsof the Bridgman method can be used in producing crystals of the presentinvention. In certain embodiments, the vertical Bridgman method was usedin producing crystals examined and discussed below.

Cs₂LiLaBr₆: Single crystals of this material were grown by the Bridgmantechnique in vertical silica ampoules under vacuum. Starting materialswere CsBr (Aldrich, anhydrous, 99.9%), LiBr, (Aldrich, anhydrous,99.9%), and LaBr₃ (Aldrich, anhydrous, 99.99+%).

Cs₂LiLaCl₆: Single crystals of this material were grown by the Bridgmantechnique in vertical silica ampoules under vacuum. Starting materialswere CsCl (Aldrich, anhydrous, 99.9%), LiCl (Aldrich, anhydrous, 99.9%),and LaCl₃ (Aldrich, anhydrous, 99.99+%).

Cs₂LiLaI₆: Single crystals of this material were grown by the Bridgmantechnique in vertical silica ampoules under vacuum. Starting materialswere CsI (Aldrich, anhydrous, 99.9%), LiI (Aldrich, anhydrous, 99.9%),and Lab (Aldrich, anhydrous, 99.99+%).

Cs₂LiLuI₆: Single crystals of this material were grown by the Bridgmantechnique in vertical silica ampoules under vacuum. Starting materialswere CsI (Aldrich, anhydrous, 99.9%), LiI (Aldrich, anhydrous, 99.9%),and LuI₃ (Aldrich, anhydrous, 99.99+%).

Scintillation Properties of Quaternary Compound Scintillators

Scintillation properties of small Bridgman grown quaternary compoundcrystals (≦300 mm³) have been characterized. This investigation involvedmeasurement of the light output, the emission spectrum, and thescintillation decay time of the crystals. Energy resolution of samplecrystals and their proportionality of response were also measured.

1. Light Output and Energy Resolution

As shown in FIGS. 1A through 1D, the energy resolution of the 662 keVphotopeak recorded with the scintillator compositions has been measuredto be in the vicinity of 5.9%, 6.1%, 6.2%, and 19% (FWHM) at roomtemperature for Cs₂LiLaBr₆:Ce, Cs₂LiLaCl₆:Ce, Cs₂LiLaI₆:0.5% Ce, andCs₂LiLuI₆:2% Ce, respectively. The light output of scintillatorcomposition crystals was measured by comparing their response to 662 keVγ-rays (¹³⁷Cs source) to the response of a BGO scintillator to the sameisotope (see FIGS. 1A through 1C). This measurement involved opticalcoupling of a scintillator crystal to a photomultiplier tube (withmulti-alkali S-20 photocathode), irradiating the scintillator with 662keV photons, and recording the resulting pulse height spectrum. In orderto maximize light collection, the scintillator composition crystal waswrapped in reflective, white Teflon tape on all faces (except the onecoupled to the PMT). An index matching silicone fluid was also used atthe PMT-scintillator interface. A pulse height spectrum was recordedwith a scintillator composition crystal. This experiment was thenrepeated with a BGO scintillator. Comparison of the photopeak positionobtained with the scintillator composition for 662 keV photon energy tothat with BGO provided estimation of light output for the scintillatorcomposition crystal. FIGS. 1A through 1D show the pulse height spectrafor a scintillator composition and for BGO under ¹³⁷Cs irradiation andamplifier shaping time of 4.0 μs. This shaping time is long enough toallow full light collection from both the scintillators. The PMT biasand amplifier gain were the same for both spectra. Based on the recordedphotopeak positions for each scintillator composition and BGO, lightoutput of Cs₂LiLaBr₆:Ce, Cs₂LiLaCl₆:Ce, Cs₂LiLaI₆:0.5% Ce, andCs₂LiLuI₆:2% Ce crystals was estimated to be about 46,000 photons/MeV,34,000 photons/MeV, 47,000 photons/MeV, and 11,000 photons/MeV,respectively. FIGS. 2A and 2B depict ²⁵²Cf and ¹³⁷Cs energy spectra forCs₂LiLaBr₆:Ce and Cs₂LiLaCl₆: Ce, respectively.

2. Emission Spectrum

Normalized emission spectra for the scintillator compositions are shownin FIGS. 3A through 3D. The scintillator composition samples wereexcited with radiation from a Philips X-ray tube having a Cu target,with power settings of 40 kVp and 20 mA. The scintillation light waspassed through a McPherson monochromator and detected by aphotomultiplier tube. The barycenter of emission for the Cs₂LiLaBr₆:Ce,Cs₂LiLaCl₆:Ce, Cs₂LiLaI₆:0.5% Ce, and Cs₂LiLuI₆:Ce samples was atapproximately 410 nm, 395 nm, 450 nm, and 490 nm, respectively. Peakemission values are shown in Table I. Emission wavelengths in this rangeare attractive for gamma-ray spectroscopy because they match well withthe spectral response of the photomultiplier tubes as well as a newgeneration of silicon photodiodes.

3. Time Profiles

FIGS. 4A through 4D show the time profiles recorded for Cs₂LiLaBr₆:Ce,Cs₂LiLaCl₆:Ce, Cs₂LiLaI₆:Ce, and Cs₂LiLuI₆:Ce samples, respectively.Time profiles of the scintillator compositions have been measured undergamma ray excitation using the delayed coincidence method (Bollinger andThomas, Rev. Sci. Instr. 32:1044 (1961)) or with a digital oscilloscope.In FIG. 4A, rise time for Cs₂LiLaBr₆:Ce were ˜0.5 ns (τ_(r1)) and ˜12 ns(τ_(r2)) and decay time constants were ˜1 ns (τ_(d1)) and 965 ns(τ_(d2)). In FIG. 4B, rise time for Cs₂LiLaCl₆:Ce were ˜0.5 ns (τ_(r1))and ˜16 ns (τ_(r2)) and decay time constants were ˜1.5 ns (τ_(d1)) and90 ns (τ_(d2)). In FIG. 4C, the rise time for Cs₂LiLaI₆:Ce was ˜2 ns(τ_(r)) and the decay time constant was ˜70 ns (τ_(d)). In FIG. 4D, therise time for Cs₂LiLuI₆:Ce was ˜2 ns and decay time constants were ˜5 ns(τ_(d1)), ˜45 ns (τ_(d2)), and ˜450 ns (τ_(d3)).

FIGS. 5A and B show time profiles for Cs₂LiLaBr₆:Ce and Cs₂LiLaCl₆:Cescintillators, respectively, where the scintillators exhibit differentdecay times for gamma rays and neutrons.

4. Non-Proportionality

As shown in FIGS. 6A through 6C, the non-proportionality ofCs₂LiLaBr₆:1% Ce, Cs₂LiLaCl₆:Ce, and Cs₂LiLaI₆:0.2% Ce scintillatorcompositions was evaluated, respectively. Non-proportionality (as afunction of energy) in light yield can be one of the important reasonsfor degradation in energy resolution of established scintillators suchas NaI(Tl) and CsI(Tl) (Dorenbos et al., IEEE Trans. Nuc. Sci. 42:2190(1995)). Light output of the scintillator compositions was measuredunder excitation from isotopes such as ²⁴¹Am (60 keV γ-rays), ⁵⁷Co(14.4, 122, and 136 keV γ-rays), ²²Na (511 keV and 1275 keV γ-rays) and¹³⁷Cs (662 keV γ-rays). The test crystals were wrapped in Teflon tapeand coupled to a PMT. Pulse height measurements were performed usingstandard NIM equipment with the scintillator exposed to differentradioisotopes. The same settings were used for the PMT and pulseprocessing electronics for each isotope. From the measured peak positionand the known γ-ray energy for each isotope, the light output (inphotons/MeV) at each γ-ray energy was estimated. The data points werethen normalized with respect to the light output value at 662 keV energyand the results (shown in FIGS. 6A through 6C) indicated thatCs₂LiLaBr₆:1% Ce, Cs₂LiLaCl₆:Ce, and Cs₂LiLaI₆:0.2% Ce were veryproportional scintillators. Over the energy range from about 60 to about1275 keV, the non-proportionality in light yield was better than or lessthan about 8%, typically about 4% or better (for corresponding valuesfor other established scintillators see, e.g., Guillot-Noel et al., IEEETrans. Nuc. Sci 46: 1274-1284 (1999)).

Overall, these measurements indicated that the quaternary compoundscintillators as described in the present invention have high lightoutput, fast response and show exceptional qualities in terms of lightoutput, energy resolution, speed and proportionality of response. Thosethat contain a neutron absorber also permit gamma ray-neutrondiscrimination by pulse-shape analysis.

Although the invention has been described with reference to the aboveexamples, it will be understood that modifications and variations areencompassed within the spirit and scope of the invention. Accordingly,the invention is limited only by the following claims along with theirfull scope of equivalents. All publications and patent documents citedin this application are incorporated by reference in their entirety forall purposes to the same extent as if each individual publication orpatent document were so individually denoted.

What is claimed is:
 1. A method of performing radiation detectioncomprising: providing a detection device including a detector assemblycomprising a mixed halide scintillator composition, the mixed halidescintillator composition comprising at least two different halidecompounds, wherein the first halide compound comprises Cs, Li and ahalide and a second halide compound comprises Cs, Li and a halide;providing a photodetector assembly optically coupled to the scintillatorcomposition; and positioning the device such that a radiation source iswithin a field of view of the scintillator composition so as to detectemissions from the source.
 2. The method of claim 1, wherein a patientis positioned between the radiation source and the scintillatorcomposition.
 3. The method of claim 1, wherein the radiation sourcecomprises a patient.
 4. The method of claim 1, wherein the emissionscomprise gamma-ray or X-ray emissions.
 5. The method of claim 4, whereinthe emissions further comprise neutron emissions.
 6. The method of claim1, wherein the emissions comprise neutron emissions.
 7. The method ofclaim 1, wherein at least one of the halide compounds comprises Cs, Liand Cl.
 8. The method of claim 1, wherein at least one of the halidecompounds comprises Cs, Li and Br.
 9. The method of claim 1, wherein atleast one of the halide compounds comprises Cs₂LiLaCl₆.
 10. The methodof claim 1, wherein at least one of the halide compounds comprisesCs₂LiLaBr₆.
 11. The method of claim 1, wherein at least one of thehalide compounds further comprising a dopant.